This invention relates to epitaxial growth processes and, more particularly, to liquid phase epitaxy (LPE) using slider apparatus.
In the prior art, horizontal growth of GaAs is commonly accomplished using a graphite slider apparatus and ramp-cooling techniques. U.S. Pat. No. 3,741,825 granted to H. F. Lockwood and M. Ettenberg on June 16, 1973 is illustrative of both the apparatus and technique. Briefly, the apparatus comprises a solution holder or boat having a plurality of tandem wells for carrying source solutions. A slider having at least two recesses in tandem, one for carrying a substrate and another for carrying a saturation seed, is inserted in a channel which extends horizontally through the boat and beneath the wells. The saturation seed precedes the substrate under each solution. Thus, in operation the boat is loaded with appropriate source solutions (e.g., Ga solutions of GaAs), and the slider is loaded with the substrate and saturation seed. The apparatus is then placed in a quartz tube within a furnace. After suitable flushing of the ambient with hydrogen, the furnace temperature is raised to a temperature at which the source solutions are saturated (about 800.degree. C for GaAs). A controlled cooling program is then instituted and the slider is moved until the saturation seed is located under the first well to establish local liquidus equilibrium. Then the slider is again moved until the saturation seed is under the second well and the substrate is under the first well. Epitaxial growth takes place on the substrate at a rate determined by the cooling rate. Simultaneously the saturation seed establishes local liquidus equilibrium at the bottom of the second solution. Repetition of these steps results in the growth of multilayered structures such as GaAs-AlGaAs double heterostructure junction lasers.
One problem consistently met in this prior art LPE technique is excessive edge growth. That is, the epitaxial layers grow at a faster rate near the edges of th substrate or wafer than at the interior portions thereof. In this regard the term edge need not be the actual outer edge of the substrate. Because the substrate is typically larger than the bottom of the source solution wells, the edge is commonly the region underlying or near to the interior vertical walls of the well. Generally, the amount of edge growth is a function of such parameters as the cooling rate and the boat design. Thus, higher cooling rates and thinner walled boats (often used to maintain close thermal contact with the furnace) both tend to exacerbate the problem. From a quantitative standpoint, thickness profiles of epitaxial layers commonly exhibit thicknesses at the edges which exceed that in central portions of the wafer by an order of magnitude. For example, in one case of a four-layer GaAs-AlGaAs double heterostructure, the thickness of the central portio was about 3.6 .mu.m whereas the peak thickness at the edges was about 58.4 .mu.m. In other similar cases the edges were as high as 170 .mu.m.
Excessive edge growth presents two significant difficulties in the prior art ramp-cooled LPE technique. First, because the edge growth extends inwardly on the wafer, the usable area of the wafer for making devices is reduced. While the per cent of usable area lost depends on the size of the wells used, in a typical example the wells are 12 mm square and the edge growth occupies a 1.5 mm band around the periphery. Thus, the usable area is reduced from 144 mm.sup.2 to about 81 mm.sup.2, a 44 percent reduction. Secondly, it is difficult to wipe off the source solution over such an irregularly shaped edge. Moreover, when the slider is spaced close to the bottom of the wells to improve wipe-off, the high edges of the wafer scratch graphite particles from the boat. These particles, as well as broken-off pieces of the edge growth, disrupt subsequent layer growth in the places where they lie.
Maintenance of nearly complete wipe-off (greater than 95 percent of the growth area) is ordinarily achieved with great difficulty. Satisfactory wipe-off is usually a compromise between nearly complete wipe-off achieved at close boat-to-slider spacings resulting in many scratches and pits on the layer surface and not-so-complete wipe-off (about 60 percent of the growth area) with fewer scratches and pits achieved at larger boat-to-slider spacings. So, one empirically adjusts the boat-to-slider spacing very carefully to a certain value for each particular LPE system.